Electromagnet with special force displacement characteristics



Sept. 22, 1959 E. J. FIELD 9 5 ELECTROMAGNET WITH SPECIAL FORCE DISPLACEMENT CHARACTERISTICS Filed larch 14. 1956 2 Sheets-Sheet l IN V EN TOR. '/J Eva-arr?- F1420 BY W, 5&4

Sept. 22, 1959 E. J. FIELD 2,905,372

ELECTROMAGNET WITH SPECIAL FORCE DISPLACEMENT CHARACTERISTICS Filed March 14. 1956 2 Sheets-Sheet 2 N\\\\\\\\\\\\\\\\\\\\& \\\\\\N\\% m ""I'I'QI'AIO ELECTROMAGNET WITH SIECIAL FORCE DISPLACEMENT CHARACTERISTICS Everett J. Field, Jeannette, Pa., assignor to r-r-n Circuit Breaker Company, Philadelphia, Pin, a corporation/of Pennsylvania Application March 14, 1956, Serial No. 511,545 9 Claims. or. 317-199 termined point of the characteristic curve as well as a relatively low impedance toward the latter part of the operating stroke.

In the construction of electromagnets, it has been customary to surround or house the solenoid coil with magnetic material so as to reduce leakage flux by reducing the reluctance of the circuit to increase the magnetic pull. However, many of these prior art structures, although achieving a relatively high initial magnetic force on the plunger or armature, nevertheless, impart such a large magnitude of energy to the plunger that the high velocity of the plunger or armature at the end of its stroke could cause damage to the mechanism. That is, there is a large impact as a result of a continuous driving force throughout the movement of the plunger or armature, and hence, the kinetic energy inherent in the rapidly moving mass is damaging.

A specific situation in which this characteristic has resulted in extreme disadvantage has been in the utilization of electromagnets in automatic reclosers, for example in the type shown in Patent No. 2,738,394, assigned to the assignee' of the instant invention. In the automatic recloser, the contacts are biased closed. The plunger or armature is directly connected to the movable contact. The solenoid or coil is energized by the fault current and hence the fault current energy is utilized to open the contacts.

Although it is desirable to have rapid separation of the contacts on the occurrence of a fault current condition, there have been situations in which the'operating mechanism of the automatic recloser has been damaged as a result of the large magnitude of kinetic energy which is inherent in the rapidly moving contact and plunger. As noted, this disadvantage arises due to the continuance of the magnetic pull or driving force on the plunger and moving contact structure throughout the entire stroke of the moving contact structure.

I have invented a novel electromagnet wherein by space proportioning and configuration of the armature it 18 possible to achieve a sharp cut-01f in the magnetic force at a designated point on the movement of the armature to its neutral position. In my invention the location of the force cut-oil point can be predetermined and set at any desired point within the movement of contact separation.

In the application of an electromagnetic tovan automatic recloser there is not a fixed magnitude of ampere turns since the solenoidor coil is energized by the fault current. Thus, although under some conditions the solenoid or operating coil may be energized by a very large magnitude of current flowing by result of severe short circuit-current, there will nevertheless be other situations in whicha mere overcurrent flows through the circuit and still requires the contacts to be separated.

2,905,812 Patented Sept. 22, 1959 In this situation, if a sharp cut-oif in the magnetic force is achieved during the opening stroke of the plunger and moving contact, it is desired to have a secondary plateau or residual force following the cutotf of the large initial magnetic force so that the entire structure will at least be carried to the latched position although not damaging any of the mechanism of the recloser.

In the use of an electromagnet in an automatic re-' closure, the solenoid or operating coil of the electromagnet is referred to as the operating coil. As above noted it is the magnet pull resulting from the energization of the operating coil which results in the movement of the armature and the movable contact structure attached thereto. Since the operating coil is energized by fault current, it is undesirable to have the operating coil in the circuit which continuously carried load current since this would of necessity introduce a relatively high impedance in the load circuit under normal conditions. To avoid this it is well known in the prior art to provide bypass contacts for the operating coil which are controlled by a relatively small control coil which is electrically connected in series with the source and load. Thus, although continuous load current flows through the control coil it introduces a relatively small magnitude of.

impedance in the circuit since it only requires a small number of'turns to operate the bypass or shunt contacts. Thus, on the occurrence of a fault current, the control coil is initially energized thereby opening the bypass contacts of the operating coil so that all of the fault current is shunted to the operating coil.

In the prior art arrangement, when the operating coil is thus energized, the rapid movement of the armature into the operating coil results in a very sharp and rapid increase of the impedance of the operating coil and thus a very large voltage is impressed across the bypass contacts. In some instances, this high voltage results in the breakdown-of these bypass contacts thereby resulting in faulty operation of the automatic recloser. In order to avoid this undesirable condition it has been necessary in the past to derate the automatic recloser. The impedance of the solenoid or operating coil at this crucial period in the operation. of the unit is a function of both the speed of operation and the quantity of magnetic material of the armature which is moved into the solenoid or. operating coil. 1

With my novel electromagnet, the proportions and construction of the armature is such that there is a considerable reduction in the quantity of magnetic material moved into the solenoid so that both therate of rise and the magnitude of ultimate impedance of the solenoid is considerably reduced, thereby eliminating the necessity of derating the automatic recloser when my novel electromagnet is used in conjunction therewith.

My novel electromagnet is provided with an armature having a stepped section wherein one portion has a relatively large cross-sectional area and a second portion having a relatively small cross-sectional area. The

solenoid or operating coil is provided withan annular magnetic or iron plate at the end which receives the armature to thereby concentrate the magnetic fiux into only one portion ofthe armature. When the solenoid or operating coil is initially energized substantially all of the flux will be concentrated into that portion of the armature which has a relatively large cross-sectional area. Hence, the magnetic pull on the armature will be relatively large since the force on the'armature will be a function of the net flux and cross-sectional areaof the The reduced cross-sectional area in the lower section of the armature is such that it will immediately be saturated by the magnetic flux concentrated therein by the annular magnetic plate. Thus, there will be a very sharp cutoff of the magnetic force. That is, since the magnetic pull is a function not only of the net 'fiux but also the cross-sectional area of the armature, there will be a sharp reduction in the magnetic pull at the instant the reduced cross-sectional area is brought into alignment with the annular magnetic plate where all of the fiux is concentrated. Thus, there is a desirable large initial pull which is cut-off after the contacts are separated.

However, it is still desirable, as above noted, to have a small residual force exerted on the armature so that it can pull the movable magnetic structure at least to the latched position. To this end the reduced cross-sectional portion of the armature provides the secondary plateau or small residual force to achieve this result. This small residual force will continue until the magnetic center of the armature structure is in alignment with the magnetic center of the solenoid and its annular magnetic plate. At this point there will be a so called neutral polarity. In the event the armature should continue to move past this neutral polarity, there will be a small downward force tending to bring it back to the neutral position.

With my novel electromagnet the step in the armature separating the section of wide and thin wall sections of the armature will provide a positive means of establishing an abrupt force cut-olf at a predetermined point of the characteristic curve. Furthermore, since one section of the armature has thin walls or a reduced crosssectional area, the magnitude of the impedance of'the solenoid or operating coil will not rise to the same magnitude as in the case when the armature has a large crosssectional area throughout. Thus, it will not be necessary to derate the automatic recloser when my novel electromagnet is used therein.

Accordingly, a primary object of my invention is to provide a novel electromagnet wherein positive means are used for establishing an abrupt cut-off at a predetermined point of the characteristic curve.

Another object of my invention is to provide an electromagnet wherein the armature has a sharp discontinuity so that special force displacement characteristics can be achieved therewith.

A still further object of my invention is the provision of electromagnetic means wherein there is an initial high magnetic pull followed by an abrupt and sharp cut-off to asmall residual force.

Another object is to provide an annular magnetic plate for the solenoid of an electromagnet together with an armature having two sections of different cross-sectional area.

A still further object of my invention is to provide an electromagnetic structure wherein the armature has one portion which may be unsaturated during the energization of the solenoid and a second section which will he saturated during energization of the solenoid.

A still further object of my invention is the provision of an armature having portions with unequal cross-sectional areas to achieve abrupt magnetic force cut-ofi following energization of the electromagnet.

Another object is to provide an electromagnetic structure wherein there is a predetermined forced cut-off type in the forced displacement characteristics of the unit.

Another object is the provision of an electromagnetic structure wherein there is a relatively constant residual force of low magnitude following the cut-off point.

A still further object is to provide an electromagnetic structure wherein a residual force may be varied from a maximum magnitude to zero and to a reverse magnitude and also means to establish the residual force at any desired level within the range of operation. I

A still further object of my invention is to provide necessary to derate maximum fault capacity of the unit when used in a biased closed circuit interrupter.

Another object of my invention is the provision of an armature having a reduced cross-sectional area thereby reducing the impedance of the solenoid or operating coil when the armature is drawn therein.

' Another object of my invention is to provide an electromagnet for use in an automatic recloser in which the armature has a section with a small cross-sectional area .thereby avoiding the necessity of derating the unit.

These and other objects of my invention will become apparent from the following description when taken in connection with the drawings in which:

Figure 1 is a cross-sectional view of an automatic recloser utilizing my novel electromagnet having special force displacement characteristics.

Figure 1a is a schematic electrical diagram of the automatic recloser illustrated in Figure 1.

Figure 2 is a schematic view of Figure 1 illustrating the construction of my novel electromagnet.

Figure 3 is an armature displacement curve vs. magnetic force illustrating the sharp force cut-oft" and small residual force of my novel electromagnet.

Referring to the figures, I have shown my novel electromagnet used in conjunction with an automatic recloser. The entire unit shown in Figures 1 and 2 would ordinarily be enclosed within a tank filled with oil.

The recloser is comprised of a chamber 10 in which are contained the cooperating contacts 11-14. The stationary contacts 11 supported on members 12 contained within the conducting member 13 are in pressure contact with the movable contact member 14. The movable contact 14 is biased to the closed position by means of the spring 15. My novel electromagnetic structure is comprised of the solenoid 17 which is the main operating coil of the automatic recloser. A magnetic annular disc 19 is positioned at the lower end of the solenoid coil 17 and serves to concentrate the flux at that point. A phenolic washer 18 may be provided to separate the solenoid 17 from the magnetic disc 19.

The electromagnetic structure has an armature 20, which is keyed by means of the ring 29 to the movable contact 14, and has two sections, an upper section 21 having a relatively large cross-sectional area as indicated by the dimension C of Figure 2 with a length A extending below the annular magnetic plate 19, and a second section 22 which has a relatively small cross-sectional area as indicated by the dimension D and having a length B.

The armature 20 is positioned with respect to the sole noid 17 so that there is an initial overlap of the armature 20 and the solenoid 17 when the armature 20 is in its neutral position or rest position (i.e. contacts closed) as seen. in Figures 1 and 2. The armature 20 may be provided with a flange 23 of non-magnetic material. The flange 23 is properly designed with respect to the chamber 24 so that the combination will serve as a hydraulic decelerating dashpot. The electrical circuit for the automatic recloser of Figure 1 is illustrated in Figure 1a wherein the continuous load current would flow from the terminal 13 through the stationary contacts 11, 12 to the movable contact means 14 through a pigtail 25 through the bypass or shunt contacts 26 of the solenoid or operating coil 17 to the control coil 27 and then to 55 the terminal 28. On the occurrence of a fault current,

the control coil 27 will be energized to thereby separate the bypass' or shunt contacts 26 so that all of the fault current will flowthrough the operating coil 17. Energization of the operating coil 17 will attract the armature 0 20 which, since it is keyed at 29 to the movable contact 14, it will result in the disengagement of the cooperating contacts 14 and 11, 12.

Thus, the solenoid or operating coil 17 is energized by fault current. Thus the ampere turns results in a maga low impedance electromagnetic means so that it is unnetic pull to attract the armautre 20 and the movable tion of the magnetic tlux into the upper section 21 of the armature 20. That is, the solenoid or operating coil 17 initially sees only the upper portion 21 of the armature which has a relatively large cross-sectional area indicated by dimension C. Since the magnetic pull is a function of the net flux and a cross-sectional area of the armature, there will be a relatively large initial pull as is illustrated in Figure 3. This large initial force will continue as the armature travels a distance A, which is the length of the upper cross-section 21 of the armaturc'20 which exists below the annular magnetic plate 19. Thus, the relatively large magnetic force indicated as approximately 80 units in the force displacement diagram of Figure 3 will continue for a distance at least equal to A.

As illustrated, the armature has a sharp discontinuity or shelf existing between upper section 21 and lower section 22. Thus, the thin wall or narrow cross-section portion of the armature'in the lower section 22, indicated by the dimension D, will result in a very sharp cut-off of the magnetic force when the shelf 30 comes into alignment with the annular magnetic plate 19. This shelf and abrupt force cut-off after travellinga distance A is illustrated in Figure 3. Since the cross-sectional area D of the lower section 21 is considerably smaller than the large cross-sectional area C of the upper section 21 and is so dimensionedthat the lower section D'will become saturated as soon as the shelf 30 comes into alignment with the annular magnetic plate 19.

As. above noted, since the magnetic pull is not only a function of the net flux but also of the cross-sectional area, the alignment of the shelf 30 with the annular magnetic plate 19 will result in a very sharp cut-E of the .force. Thus, after the armature has travelled a distance A the flux will be concentrated into the small cross-sectional area D of the lower section 22 of armature 20 so that there will now be only a small residual force D which is determined by dimension D, as illustrated in Figure 3. This small residual magnetic force D'- is de sirable in many applications of an electromagnet and especially desirable when used in an automatic recloser. That is in an automatic recloser it is necessary to not only rapidly separate the contacts but also necessary to ensure that there is a force on the moving structure to at least bring it to the latch open position. The small re sidual force indicated by D in Figure 3 will ensure this latching operation. The small residual force, resulting from the small cross-sectional area D of the lower section 22 of the armature, will continue to be exerted until the magnetic center of the armature 20 is in alignment with the magnetic center of the solenoid 17 and its annular magnetic plate 19.

That is, the solenoid and plunger of an attractive electromagnet is a differential device wherein the net opposing mechanical forces balance each other when neutral polarity of the solenoid coincide. In the event the magnetic center of the armature or plunger 20 should move up and past the magnetic center of the solenoid 17 and its magnetic plate 19, then there will be a net downward force on the armature 20 as is illustrated in Figure 3.

ot the upper section 21 existing below the magnetic plate 19, the magnitude of the small residual force will be a function of the small cross-sectional area D of the lower section 22 and will exist for a period of time depending upon length B and its relation to the magnetic center of the armature 20 and magnetic center of the solenoid 17 and its annular magnetic plate 19. Thus there is a well defined predetermined force cut-oi! and force displace ment characteristic and a relatively constant residual force of low magnitude following cut-oil point. By modifying dimensions A, B, C, D it is possible to modify the force cut-oif point. as well as the magnitude and duration of the residual force.

In the prior art arrangement, the electromagnetic system usually remains energized, and hence, continues to accelerate the moving'masses through the entire structure. Thus there is a very large impact on the operating mechanism as a result of the persistence of the driving force throughout the contact separating structure. This kinetic energy inherent in the rapidly moving parts is frequently damaging to the mechanism. However, in my arrangement the sharp force cut-ofi avoids this disadvantage. ,This is achieved with my novel unit even though the magnitude of the attractive force near the end of the stroke is considerably reduced because of sharp or abrupt cutoflf in force at a predetermined point. This desirable feature is achieved without decreasing the initial acceleration or attracting force on the armature.

As has heretofore been noted in connection with Figure 1a, the type of electromagnet to which my invention is directedcan be used in automatic interrupting equip ment, especially the type which is biased closed and utilizes the fault current energy for separating the contacts. In these arrangements it is usually necessary to provide bypass contacts 26 for the operating coil 17. However, during the opening operation of the automatic recloser, all-of the magnetic material of the armature is brought into the solenoid or operating coil. This results in a very rapid rise to.a large impedance which results in an induced voltage which may cause the bypass or shunt contacts to break down. Previously it has been necessary to derate automatic interrupting equipment due to this very undesirable feature. However, in my novel arrangement the armature 20 has a sharp demarcation 30 thereby resulting in the lower section 22 having a relatively small cross'sectional area D. Hence, the amount of magnetic material brought into the solenoid 17 is considerably reduced thereby reducing the impedance of this coil during its energization by fault current. Since the amount of magnetic material is considerably reduced thereby also reducing the induced voltage which will be impressed across the bypass or shunt contacts'26. Thus, this lower impedance of the assembly now makes it unnecessary to derate the maximum fault capacity when my novel electromagnet is used in an automatic circuit interrupter.

Thus, I have provided a novel electromagnetic structure wherein positive means are provided for establishing an abrupt force cut-off at any predetermined point in the force displacement characteristic of the device and furthermore'which has an inherent low impedance thereby making it unnecessary to derate the maximum fault capacity when the structure is used in automatic circuit interrupting equipment.

In the foregoing, I have described my invention only in connection with preferred embodiments thereof. Many variations and modifications of the principles of my invention within the scope of the description herein are ob vious. Accordingly, I prefer to be bound not by the specific disclosure herein but only by the appending claims.

I claim:

1. An electromagnetic device being comprised of a solenoid, an armature and an annular magnetic plate; said solenoid and said annular magnetic plate surrounding said armature; said annular magnetic plate positioned at one end of said solenoid, said armature being comprised of a first and second section having different cross-sectional areas; said first section having a larger cross-sectional area than said second section, said annular magnetic plate adjacent to and surrounding only said first section when said armature is in a rest position, said second section being operatively positioned so as to be the last part of said armature to be drawn within said annular magnetic plate and said solenoid when said solenoid is energized.

2. An electromagnetic device being comprised of a solenoid, an armature and an annular magnetic plate; said solenoid and said annular magnetic plate surrounding said armature; said annular magnetic plate positioned at one end of said solenoid; said armature having a first and second section separated by a sharp demarcation whereby said first section has relatively large cross-sectional area and said second section has a relatively small crosssectional area with respect to each other, said annular magnetic plate being operatively positioned with respect to said armature to concentrate flux only in said first section when said solenoid is initially energized.

3. An electromagnetic device being comprised of a solenoid, an armature and an annular magnetic plate; said solenoid and said annular magnetic plate surrounding said armature; said annular magnetic plate positioned at one end of said solenoid; said armature having a first and second section separated by a sharp demarcation whereby said first section has a relatively large cross-sectional area and said second section has a relatively small cross-sectional area with respect to each other, said annular magnetic plate operatively positioned with respect to said armature to concentrate flux solely in said first section when said solenoid is initially energized; and to concentrate flux solely in said second section when energization of said solenoid has moved said armature a predetermined distance; said first section having a suliiciently large cross-sectional area so that it requires much more flux than said second section requires for saturation, said second section having a sufficiently small cross-sectional area so as to be saturated by the net flux when said solenoid is energized by the rated current.

4. An electromagnetic device being comprised of a solenoid, an armature and an annularmagnetic plate; said solenoid and said annular magnetic plate surrounding said armature; said annular magnetic plate positioned at one end of said solenoid; said armature have a first and second section separated by a sharp demarcation whereby said first section has relatively large cross-sectional area and said second section has a relatively small cross-sectional area with respect to each other; said annular magnetic plate being positioned adjacent only the first section of said armature when said armature is in a rest position, said annular magnetic plate being thus operatively positioned with respect to said armature to concentrate substantially all of the magnetic flux of said solenoid only in said first section when said solenoid is initially energized to thereby result in a substantially large initial magnetic pull.

5. An electromagnetic device being comprised of a solenoid, an armature and an annular magnetic plate; said solenoid and said annular magnetic plate surrounding said armature; said annular magnetic plate positioned at one end of said solenoid; said armature having a first and second section separated by a sharp demarcation whereby said first section has relatively large cross-sectional area; and said second section has a relatively small cross-sectional area with respect to each other; said annular magnetic plate being positioned adjacent only the first section of said armature when said armature is in a rest position, said annular magnetic plate being operatively positioned with respect to said armature to concentrate substantially all of the magnetic fiux of said solenoid only in said first section when said solenoid is initially energized to thereby result in a substantially large initial magnetic pull; said relatively large initial attracting magnetic pull continuing only until said sharp demarcation is in alignment with said annular magnetic plate.

6. An electromagnetic device being comprised of a solenoid, an armature and an annular magnetic plate;

said solenoid and said annular magnetic plate surroundsectional area andsaid second section has a'v relatively small cross-sectional area with respect to each other; said annular magnetic plate being positioned adjacent only the first section of said armature when said armature is in a rest position, said annular magnetic plate being operatively positioned with respect to said armature to concentrate substantially all of the magnetic flux of said solenoid only in said first section when said solenoid is initially energized to thereby result in a substantially large initial magnetic pull; said relatively large initial attracting magnetic pull continuing only until said sharp demarcation is in alignment with said annular magnetic plate; said secondsection of said armature having a sufficiently small cross-sectional area to be saturated by the net flux of said solenoid when energized by its rated current; said annular magnetic plate and said sharp demarcation being operatively positioned with respect to each other to produce an abrupt decrease in magnetic pull on said armature due to said saturable small cross-sectional area of said second section of said armature, at the moment said sharp demarcation reaches said alignment with said annular magnetic plate.

7. An electromagnetic device being comprised of a solenoid, a flat annular magnetic plate lying in a thin plane, and an armature; said solenoid and said annular magnetic plate surrounding said armature, said armature having two elongated sections of constant but unequal cross-sectional area, with a sharp demarcation between said sections, the first of said sections having the larger cross-sectional area, said cross-sectional area being sufliciently greater in magnitude than the cross-sectional area of the second of said sections to require much more flux than said second section requires for saturation, said second section having a cross-sectional area of sufficiently small magnitude to be saturated by the flux generated when said solenoid is energized by its rated current, said flat annular magnetic plate being operatively positioned at one end of said solenoid to concentrate the flux of said solenoid in said thin plane of said annular magnetic plate, said armature being operatively positioned with respect to said annular magnetic plate to have only said first section of said armature projecting perpendicularly through said thin plane of flux concentration when said solenoid is initially energized, said armature being further operatively positioned with respect to said annular magnetic plate to require, in the following order, that said first section of said armature pass beyond said thin plane .of flux concentration, that said sharp demarcation thereafter pass perpendicularly through and beyond said plane of flux concentration, and that thereafter said second section only of said armature ride perpendicularly through said thin plane of flux concentration during the remainder of the travel of said armature, when said solenoid is energized, thereby resulting in an initially large accelerating force on the armature, followed, after said demarcation passes through said plane of flux concentration, by a smaller residual force causing said armature to complete its desired travel fully but less violently.

8. An electromagnetic'device being comprised of a solenoid, an armature, and a flat annular magnetic plate lying in a thin plane, said fiat annular magnetic plate and said solenoid surrounding said armature, said flat annular magnetic plate being operatively positioned at one end of said solenoid to concentrate the flux of said solenoid in said thin plane of said annular magnetic plate, said armature having two sections of difierent but constant cross-sectional area, the section of the smallest cross-sectional area being operatively positioned with respect to said annular magnetic plate to be the only portion of said armature within said thin plane of flux concentration at a time subsequent to the initial energization of said solenoid, thereby limiting the magnitude of the impedance of said solenoid and avoiding the necessity of derating said electromagnet device.

9. An electromagnetic device comprising an armature, a solenoid, and a fiat annular magnetic plate lying in a thin plane, said solenoid and said annular magnetic plate surrounding said armature, said annular magnetic plate being operatively positioned at one end of said solenoid to concentrate the flux of said solenoid in said thin plane of said annular magnetic plate, said armature having two sharply demarcated sections, of unequal flux saturation capacity, the section of said armature having the smaller of said flux saturation capacities being saturable by a small fraction of the rated energization of said solenoid, the section of said armature having the smaller of said flux saturation capacities being operatively positioned to be the last part of said armature drawn within said solenoid and said annular magnetic plate when said solenoid is initially energized, and being further operatively positioned to be the only part of said armature within said thin plane of flux concentration at a time subsequent to the initial energization of said solenoid.

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